Friction stir welding (FSW) is a welding process which uses heat generated from high-pressure friction to form a joint between two workpieces and/or to fix cracks in a workpiece. That is, during FSW operations, an FSW tool traverses a joint or seam disposed between the workpieces (or a crack in a workpiece) and the workpiece(s) are plasticized by frictional heat generated by rotation of the FSW tool. As the FSW tool traverses the seam, the FSW tool is also pressed against the workpieces, which are fixed relative to each other during the welding operation. More specifically, an FSW tool includes a shoulder and a pin or probe that extends out from the shoulder. During the welding operation, the shoulder is pressed against the workpiece(s) and the pin rotates in the seam between the workpieces (or in a crack in a workpiece). In some FSW heads, the shoulder rotates with or relative to the pin, but in other FSW heads, the shoulder may be stationary. Rotation of the pin (and the shoulder in some instances), softens and mixes the materials forming the workpieces. Then, the mixed materials consolidate to form a solid-state weld.
The FSW tool can traverse a seam (or crack) when the welding tool moves relative to the workpiece(s) and/or when the workpiece(s) are moved relative to the welding tool (e.g., the welding tool may be stationary). Regardless, during FSW, the welding tool must be pressed against the workpieces with great force to frictionally heat the workpieces enough to cause the desired plasticizing in the seam, and it is important to know the axial force acting on the tool and workpieces to properly calibrate and control the FSW operations (and perhaps, to adjust a backing) to provide high-quality welds. Thus, typically, FSW heads include a force measuring sensor, such as a load cell, and it is important that the force measuring sensor provides accurate data.